The present invention concerns a method for treating solid waste with an organic fraction, according to which method the waste is sieved while wet.
In the case of a selected collection of domestic waste, the organic fraction, provided it does not contain any major amounts of heavy metals, can be biologically treated into a high-grade compost.
The remaining residual waste, also called grey waste, still contains an amount of organic material and materials which can be recycled. This waste is burnt or dumped, which is relatively expensive and not environmentally friendly.
In some cases there is no selected collection of the domestic waste. This mixed domestic waste, which still contains a large fraction of organic material, is treated in the same manner as the above-mentioned residual waste.
Installations have been built to recycle the fractions of the above-mentioned residual waste and mixed waste which are suitable to be recycled. A thorough pre-selection was hereby carried out, followed by a biological treatment to obtain compost.
The major problem thereby is the quality of the end products, especially the presence of heavy metals, in particular in the compost.
Up to now, the selection has not been sufficient to produce large amounts of marketable end products with a stabilised organic fraction having a low proportion of heavy metals.
For several years, also dry selection of residual waste and mixed domestic waste has been applied by means of sieves, magnets erected above conveyor belts, air selectors, etc. The quality of the end products is such that they are not suitable for re-use or recycling. The dry selection techniques are so inefficient that in many countries, a separate collection of recyclable waste and biowaste has been introduced so as to obtain separate flows and so as to obtain products of an acceptable quality thanks to selection and treatment.
According to known methods for treating grey and mixed domestic waste or other solid waste with an organic fraction, the waste is biologically treated by making use of aerobic composting. The waste is stabilised and dried while being composted, after which ferrous metals and a fine sand fraction are recycled. The other major part of the dry substance of the waste has to be burnt, which is expensive.
Wet selection offers interesting prospects, in particular for residual waste, mixed waste with an organic fraction originating from domestic waste, and industrial waste which is comparable to domestic waste, which have been pre-selected in a conventional manner.
The removal of ferrous metals by means of magnets which are built in in waste conveyor devices or which hang over them, is restricted to large parts which are attracted by the magnets. A large number of smaller particles remains embedded in the waste.
Few devices for the treatment of solid domestic waste reduce the organic fraction to less than 20 mm, since the costs therefor rise fast with the degree of reduction.
Wet selection offers the possibility to loosen the particles from one another by adding large amounts of water, as a result of which the different components become accessible for selection.
However, the application of wet selection methods has been restricted up to now, as they usually produce strongly contaminated waste water, whose further treatment is expensive.
In aerobic composting installations, in which a solid substance content of about 50% is ideal, only a small amount of water can be added before an excess of waste water is produced which cannot be internally recycled in the composting device.
Methods whereby use is made of an anaerobic fermentation offer more possibilities for a wet selection, since said fermentation takes place in more humid circumstances than the aerobic composting, and an excess of waste water often cannot be avoided, so that a device for the water removal and the treatment of waste water is in any case required.
Anaerobic fermentation of waste can take place by means of dry fermentation, with over 15% dry substance in the reactor, as well as by means of wet fermentation with less than 15% dry substance in the reactor.
Methods whereby use is made of the wet fermentation for the treatment of waste which has been selected at the source, are designed to remove pollutants such as floating materials and heavy substances before the wet fermentation takes place, such that a fraction is obtained which is rich of organic material and poor of pollutants, as described for example in EP-A-0.520.172.
According to this last method, solid components are separated before the ferrous metals are separated from the dry waste by means of magnets and after water has been added, after which the residue is subjected to the wet fermentation.
A device which is suitable for the wet selection of heavy aggregates from mixed domestic waste or similar waste is described in EP-A-0.228.724. Stones, ceramic materials, batteries and heavy lumps, including some lumps of synthetic material, are separated by means of a separating tank filled with water, before the biological treatment takes place, so as to obtain a compost with few heavy components.
The treatment of mixed solid domestic waste may also include a wet pre-treatment into a pulp or a wet sieving, followed by a separation by a hydrocyclone.
According to such a known method, the waste with an organic fraction is pre-treated first in a dry selection device, where a combustible fraction, ferrous metals and other materials are recycled.
Mixed organic waste is separated by means of sieving, and after having been diluted with water, it is supplied to hydrocyclones, where the large inert components are separated.
The remaining fine organic material is separated by means of a sieve and then directed to a hydrocyclone again, where the sand fraction is removed. This last fraction is dewatered.
The rough organic fraction which is stopped by the above-mentioned sieve, is mixed with synthetic materials and other undesirable pieces which have already been separated and reduced in a crushing mill.
The remaining organic fraction is finally subjected to a fermentation.
As the separation takes place before the fermentation, the water is strongly contaminated, however, since all soluble components, such as organic fatty acids, sugars, etc. are present in the waste. Moreover, large quantities of water are required, and the separation is inefficient due to the heterogeneous character of the waste, which still contains the sticky and smelly organic substances which will have to be degraded in the subsequent fermentation or composting.
In fact, only sand is obtained as a recyclable material.
According to other known methods, the materials are separated after the fermentation or composting of the organic fraction. U.S. Pat. No. 4,079,837 describes a method for the recycling of recyclable materials after a fragmentation by means of thermal explosive decompression, followed by a biodegradation in a composting treatment. The fragmented and composted waste residue is separated into several fractions by means of conventional dry sieving and separation by air. Synthetic material is separated by means of floatation.
Fine sand, clay and other inert materials can be separated during the treatment after the hydrolysis in a two-phase fermentation as described in EP-A-0.142.873.
All the above-mentioned methods with wet treatments produce a compost of low quality and secondary materials, most of which contain too many heavy metals, especially if domestic residual waste or mixed waste is taken as a basis.
The positive effect of the biological treatment is often mainly restricted to the recycling of energy in the form of biogas and the production of a highly calorific fraction at the time of the dry selection during the pre-treatment.
The invention aims a method for the treatment of solid waste with an organic fraction which does not have the above-mentioned disadvantages and which makes it possible to obtain end products which do not contain large proportions of heavy metals.
This aim is reached according to the invention in that a slurry of the waste which has been diluted with water is sieved, at least by means of a sieving device which lets pieces through having a dimension in at least one direction, and preferably in at least two standing directions, which is situated between 2 and 20 mm, after which, from the slurry which has passed through, a part of the ferrous metals which are present in the waste is removed from said waste by means of magnets, and the slurry is finally dewatered.
It was found that the heavy metals usually adhere to ferrous metals, so that thanks to the removal of the latter, the concentration on the rest of the waste is reduced.
The diluted slurry can be produced in a separate stage which takes place just before the sieving or even during the sieving. However, the dilution into a slurry can also take place during a pre-treatment, for example during an anaerobic fermentation or hydrolysis when the dry substance content is low.
Preferably, waste is sieved which has been diluted into a slurry with a dry substance content of less than 10%.
Between the sieving and the removal of the ferrous metals, the above-mentioned slurry can be aerated.
After the removal of the ferrous metals, the sand can be removed from the slurry, for example by means of at least one hydrocyclone, and the fibrous material can be removed from it, for example by sieving with the help of at least one sieving device which lets pieces through whose dimension in at least one direction, and preferably in at least two standing directions, is smaller than 2 mm.
Preferably, after the separation of the ferrous metals, and after the water has been removed, the dewatered slurry is subjected to a biological treatment, in particular composting.
In order to better explain the characteristics of the invention, the following preferred embodiments of a method for the treatment of solid waste containing an organic fraction according to the invention are described as an example only without being limitative in any way, with reference to the accompanying drawings, in which: